U.S. patent application number 11/292143 was filed with the patent office on 2006-07-13 for optical device, image display apparatus, and head-mounted display.
This patent application is currently assigned to KONICA MINOLTA PHOTO IMAGING, INC.. Invention is credited to Ichiro Kasai, Tetsuya Noda, Yasushi Tanijiri.
Application Number | 20060152782 11/292143 |
Document ID | / |
Family ID | 36652946 |
Filed Date | 2006-07-13 |
United States Patent
Application |
20060152782 |
Kind Code |
A1 |
Noda; Tetsuya ; et
al. |
July 13, 2006 |
Optical device, image display apparatus, and head-mounted
display
Abstract
When an optical device is produced by bonding on a transparent
base member an optical element formed as a hologram, the optical
element and the transparent base member are both formed of an
acrylic material. In particular when, as an optical device, an
eyepiece optical system is produced by holding an optical element
between two transparent base members, the optical element, the
transparent base members, and the adhesive with which the
transparent base members are joined together are all formed of an
acrylic material. In this way, by building an eyepiece optical
system with a similar kind of material, namely an acrylic material,
it is possible to obtain increased adhesion among the materials of
the individual components of the eyepiece optical system without
performing special processing. Moreover, the transparent base
members formed of an acrylic material more securely absorb shock
and external pressure than those formed of glass.
Inventors: |
Noda; Tetsuya; (Tenri-shi,
JP) ; Tanijiri; Yasushi; (Osakasayama-shi, JP)
; Kasai; Ichiro; (Toyonaka-shi, JP) |
Correspondence
Address: |
SIDLEY AUSTIN LLP
717 NORTH HARWOOD
SUITE 3400
DALLAS
TX
75201
US
|
Assignee: |
KONICA MINOLTA PHOTO IMAGING,
INC.
|
Family ID: |
36652946 |
Appl. No.: |
11/292143 |
Filed: |
December 1, 2005 |
Current U.S.
Class: |
359/13 ;
359/14 |
Current CPC
Class: |
G03H 2270/55 20130101;
G03H 1/0272 20130101; G02B 27/0103 20130101 |
Class at
Publication: |
359/013 ;
359/014 |
International
Class: |
G03H 1/00 20060101
G03H001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2004 |
JP |
2004-351706 |
Claims
1. An optical device comprising: a transparent base member; and an
optical element formed as a hologram and bonded on the transparent
base member, wherein the optical element and the transparent base
member are both formed of an acrylic material.
2. The optical device of claim 1, wherein the transparent base
member is joined to another transparent base member formed of an
acrylic material so that the optical element is held between the
transparent base members.
3. The optical device of claim 2, wherein the transparent base
members are joined together with adhesive formed of an acrylic
material.
4. The optical device of claim 3, wherein the adhesive is of an
ultraviolet-curing type.
5. The optical device of claim 1, wherein the optical element is
formed by bonding a hologram photosensitive material in an
unexposed state to the transparent base member to be used during
reproduction and then exposing the hologram photosensitive material
to laser light.
6. The optical device of claim 1, wherein the optical element is
bonded to the transparent base member through a polymerization
reaction that takes place in an exposure process in which an
hologram photosensitive material is exposed to laser light and in a
fixing process in which the hologram photosensitive material is
fixed by being irradiated with light.
7. The optical device of claim 1, wherein the transparent base
member has a spectral transmittance of 10% or less at a wavelength
of 360 nm.
8. The optical device of claim 1, wherein the transparent base
member has a spectral transmittance of 80% or more at a wavelength
of 400 nm.
9. The optical device of claim 1, wherein a deflection temperature
under load of the transparent base member is set to be higher than
or equal to a temperature that permits unreacted monomers in the
hologram photosensitive material to diffuse and move within the
hologram photosensitive material.
10. The optical device of claim 1, wherein the optical element has
a plurality of diffraction efficiency peaks corresponding to a
plurality of different wavelengths, and a sum of diffraction
efficiency values at the different wavelengths at which the
diffraction efficiency peaks are located is 100% or more.
11. The optical device of claim 10, wherein the different
wavelengths are wavelengths corresponding to red, green, and blue,
respectively.
12. The optical device of claim 10, wherein a maximum diffraction
efficiency value among the diffraction efficiency values at the
different wavelengths at which the diffraction efficiency peaks are
located is 70% or more.
13. An image display apparatus comprising: an optical device; and
an image display element that displays an image to feed the image
to the optical device, the optical device comprising: a transparent
base member; and an optical element formed as a hologram and bonded
on the transparent base member, wherein the optical element and the
transparent base member are both formed of an acrylic material.
14. The image display apparatus of claim 13, wherein the optical
element included in the optical device is a volume-phase-type
reflective hologram.
15. The image display apparatus of claim 13, wherein the optical
element included in the optical device is a combiner that directs
to an observer's eye the image fed from the image display element
and an outside-world image simultaneously.
16. The image display apparatus of claim 13, wherein the optical
device forms an eyepiece optical system that directs to an
observer's eye an enlarged virtual image of the image displayed by
the image display element.
17. The image display apparatus of claim 16, wherein the eyepiece
optical system has a non-axisymmetric optical power.
18. The image display apparatus of claim 13, wherein the
transparent base member included in the optical device totally
reflects, within the transparent base member itself, light of the
image fed from the image display element and thereby directs the
light to the optical element.
19. The image display apparatus of claim 13, wherein the optical
element included in the optical device has a transmittance of 10%
or more.
20. A head-mounted display comprising: an image display apparatus;
and a supporter that supports the image display apparatus before an
observer's eye, the image display apparatus comprising: an optical
device; and an image display element that displays an image to feed
the image to the optical device, the optical device comprising: a
transparent base member; and an optical element formed as a
hologram and bonded on the transparent base member, wherein the
optical element and the transparent base member are both formed of
an acrylic material.
Description
[0001] This application is based on Japanese Patent Application No.
2004-351706 filed on Dec. 3, 2004, the contents of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical device having a
hologram optical element bonded on a transparent base member,
relates also to an image display apparatus employing such an
optical device, and relates further to a head-mounted display
employing such an image display apparatus.
[0004] 2. Description of Related Art
[0005] An optical element such as a hologram, a half-mirror coat,
or a beam splitter layer, when used embedded in a transparent base
member (held between two transparent base members), is not affected
by ambient factors such as humidity and oxygen. Thus, such an
optical element is very useful as a combiner in, for example, a
head-up display or a head-mounted display.
[0006] In general, a head-up display, a hologram screen, or the
like is fabricated by using a photopolymer as a hologram
photosensitive material and glass as a base member. The reasons
are, among others, that, when a photopolymer is used, unlike when a
silver halide-based material or gelatin bichromate is used, a
hologram can easily be produced by a dry process, and that glass
used as the base member is transparent and durable and permits easy
fabrication of a large-area, smooth optical surface. However, while
a photopolymer is an organic material, glass used as the base ember
is an inorganic material, and this results basically in poor
adhesion between the two materials.
[0007] Methods for increasing adhesion between a hologram and a
base member are disclosed, for example, in the following patent
publications: Japanese Patent Application Laid-open No. H11-161138
(Patent Publication 1); Japanese Patent Application Laid-open No.
H11-161142 (Patent Publication 2); and Japanese Patent Application
Laid-open No. H7-234627 (Patent Publication 3).
[0008] According to Patent Publication 1, a hologram photosensitive
material is mixed with a silane coupler to increase adhesion
between the hologram photosensitive material and a glass base
member, which is inorganic. According to Patent Publication 2, the
surface of a base member is treated with a silane coupler so that
the silane coupler increases adhesion between the base member,
which is inorganic, and a hologram photosensitive material, which
is organic. According to Patent Publication 3, after a hologram
photosensitive material is bonded to a base member, these are,
before being exposed to laser light, heated to incase adhesion
between the hologram photosensitive material and the base
member.
[0009] However, in a case where the base member to which the
hologram photosensitive material is bonded is formed of glass, to
increase adhesion between the hologram photosensitive material and
the base member, it is necessary, as described above, to perform
extra preprocessing, such as preprocessing for mixing the hologram
photosensitive material with another substance or special
processing for treating the surface of the base member. This makes
a hologram optical element difficult to fabricate. Moreover, in a
case where a hologram optical element so fabricated is used as a
combiner in a head-mounted display, the use of glass as the base
material diminishes safety to the eye.
[0010] On the other hand, in the method according to Patent
Publication 3, the heating performed before the exposure to laser
light may cause the monomer contained in the hologram
photosensitive material to react before being exposed to laser
light, possibly resulting in smaller-than-expected refractive index
modulation through laser exposure.
SUMMARY OF THE INVENTION
[0011] In view of the conventionally encountered inconveniences
discussed above, it is an object of the present invention to
provide an optical device that offers increased adhesion between a
hologram photosensitive material and a base member without
requiring special processing and that offers higher safety, to
provide an image display apparatus incorporating such an optical
device, and to provide a head-mounted display incorporating such an
image display apparatus.
[0012] To achieve the above object, according to one aspect of the
present invention, an optical device is characterized in that it is
provided with a transparent base member and an optical element
formed as a hologram and bonded on the transparent base member, and
that the optical element and the transparent base member are both
formed of an acrylic material.
[0013] With this design, the optical element formed as a hologram
and the transparent member to which the optical element is bonded
are both formed of a similar kind of material, namely an acrylic
material. Thus, it is possible to obtain increased adhesion between
the two components without performing special processing on either
of them. Moreover, since the transparent base member is formed of
an acrylic material, it more securely absorbs shock and external
pressure than one formed of glass. Thus, even when an optical
device according to the present invention is used as a combiner in
a head-mounted display, it offers higher safety to the eye of the
observer wearing the head-mounted display.
[0014] According to another aspect of the present invention, an
image display apparatus is characterized in that it is provided
with an optical device according to the present invention as
described above and an image display element that displays an image
to feed it to the optical device. With this design, the observer
can simultaneously observe, via the optical device, the image fed
from the image display element and, also via the optical device but
here in a see-through fashion, the outside-world image.
[0015] According to still another aspect of the present invention,
a head-mounted display is characterized in that it is provided with
an image display apparatus as described above and a supporter that
supports the image display apparatus before an observer's eye. With
this design, the image display apparatus is supported before the
observer's eye by the supporter, and this permits the observer to
observe, with his or her hands free, the outside-world image and
the image displayed on the image display element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other objects and features of the present
invention will become clear through the following description of
preferred embodiments taken in conjunction with the accompanying
drawings, in which:
[0017] FIG. 1 is a diagram schematically illustrating the
production procedure of the eyepiece optical system of the image
display apparatus used in a head-mounted display embodying the
present invention;
[0018] FIG. 2A is a plan view showing an outline of the structure
of the above head-mounted display;
[0019] FIG. 2B is a side view of the above head-mounted
display;
[0020] FIG. 2C is a front view of the above head-mounted
display;
[0021] FIG. 3A is a plan view showing another example of the
structure of the above head-mounted display;
[0022] FIG. 3B is a side view of the above head-mounted
display;
[0023] FIG. 3C is a front view of the above head-mounted
display;
[0024] FIG. 4 is a sectional view showing an outline of the
structure of the above image display apparatus;
[0025] FIG. 5A is a plan view showing an outline of the structure
of one of the two transparent base members that forms the above
eyepiece optical system;
[0026] FIG. 5B is a front view of the above transparent base
member;
[0027] FIG. 5C is a plan view showing an outline of the structure
of the other transparent base member;
[0028] FIG. 5D is a front view of the above transparent base
member;
[0029] FIG. 5E is a plan view of the above eyepiece optical
system;
[0030] FIG. 5F is a sectional view of the above two transparent
base members forming the above eyepiece optical system, taken
around their joint surfaces;
[0031] FIG. 6A is a graph showing an example of the diffraction
efficiency at different wavelengths in the optical element of the
above eyepiece optical system;
[0032] FIG. 6B is a graph showing the relationship between the
wavelength and light intensity of the light source that feeds light
to the above optical element during reproduction; and
[0033] FIG. 6C is a graph showing another example of the
diffraction efficiency at different wavelengths in the above
optical element.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] Hereinafter, an embodiment of the present invention will be
described with reference to the drawings.
1. Head-Mounted Display
[0035] FIG. 2A is a plan view showing an outline of the structure
of a head-mounted display (hereinafter abbreviated to "HMD")
embodying the invention, FIG. 2B is a side view of the same HMD,
and FIG. 2C is a front view of the same HMD. The HMD includes an
image display apparatus 1 and a supporter 2 that supports it, and
has an appearance like that of common eyeglasses of which one of
the lenses (for example, the left-eye lens) has been removed.
[0036] The image display apparatus 1 permits an observer to observe
the outside-world image in a see-through fashion, and
simultaneously displays an image to feed it, as a virtual image, to
the observer. In the image display apparatus 1 shown in FIG. 2C,
the part thereof that corresponds to the right-eye lens of
eyeglasses is composed of two transparent base members 22 and 23
(see FIG. 4), which will be described later, that are bonded
together. The structure of the image display apparatus 1 will be
described in detail later.
[0037] The supporter 2 supports the image display apparatus 1
before the observer's eye (for example, the right eye), and
includes a bridge 3, frames 4, temples 5, nose pads 6, and a cable
7. The frames 4, the temples 5, and the nose pads 6 are provided in
pairs each including a left one and a right one and, wherever
distinction is necessary, they are referred to as the right frame
4R, the left frame 4L, the right temple 5R, the left temple 5L, the
right nose pad 6R, and the left nose pad 6L.
[0038] One end of the image display apparatus 1 is supported on the
bridge 3. This bridge 3 supports, in addition to the image display
apparatus 1, the left frame 4L and the nose pads 6. The left frame
4L pivotably supports the left temple 5L. The other end of the
image display apparatus 1 is supported on the right frame 4R. The
right frame 4R, at the end thereof opposite to where it supports
the image display apparatus 1, pivotably supports the right temple
5R. The cable 7 contains conductors via which external signals (for
example, image and control signals) and electric power are fed to
the image display apparatus 1, and is laid along the right frame 4R
and the right temple 5R.
[0039] When an observer uses the HMD, the observer wears it on the
head as if to wear common eyeglasses, with the right and left
temples 5R and 5L kept in contact with the right and left side
parts of the head and the nose pads 6 on the nose. In this state,
when the image display apparatus 1 displays an image, the observer
can observe, as a virtual image, the image displayed by the image
display apparatus 1, and can simultaneously observe the
outside-world image in a see-through fashion via the image display
apparatus 1.
[0040] The HMD may be designed otherwise than to include only one
image display apparatus 1. For example, FIG. 3A is a plan view
showing another example of the structure of the HMD, FIG. 3B is a
side view of the same HMD, and FIG. 3C is a front view of the same
HMD. As shown in these diagrams, the HMD may have two image display
apparatuses 1 arranged one before each eye of an observer. In this
case, the image display apparatus 1 arranged before the left eye is
supported on the bridge 3 and the left frame 4L in a space secured
between them. Moreover, the cable 7 is connected to both the image
display apparatuses 1 so that they are both fed with external
signals and the like via the cable 7.
2. Image Display Apparatus
[0041] Next, the image display apparatus 1 mentioned above will be
described in detail. FIG. 4 is a sectional view showing an outline
of the structure of the image display apparatus 1. The image
display apparatus 1 is composed of an image display element 11 and
an eyepiece optical system 21.
[0042] The image display element 11 includes a light source 12, a
one-directional diffuser plate 13, a condenser lens 14, and an LCD
15. Here, the light source 12, the one-directional diffuser plate
13, and the condenser lens 14 together form an illumination optical
system for illuminating the LCD 15.
[0043] The light source 12 is built with, for example, an RGB
hybrid LED that emits light in three wavelength bands whose center
frequencies are 465 nm, 520 nm, and 635 nm. The light source 12 may
be a white light source that emits white light.
[0044] The one-directional diffuser plate 13 diffuses the
illumination light from the one-directional diffuser plate 13 with
varying degrees of diffusion in different directions. More
specifically, the one-directional diffuser plate 13 diffuses the
light incident thereon at about 40 degrees in the direction
corresponding to the left/right direction with respect to the
observer wearing the HMD (that is, in the direction perpendicular
to the plane of FIG. 4) and at about 2 degrees in the direction
corresponding to the up/down direction with respect to the observer
wearing the HMD (that is, in the direction parallel to the plane of
FIG. 4).
[0045] The condenser lens 14 condenses the light diffused by the
one-directional diffuser plate 13. The condenser lens 14 is so
arranged as to permit the diffused light to efficiently form an
optical pupil E. The LCD 15 modulates the light incident thereon
according to an image signal and thereby displays an image.
[0046] The eyepiece optical system 21 includes two transparent base
members 22 and 23 and an optical element 24. The eyepiece optical
system 21 serves simultaneously as both an optical device that
permits the outside-world image to be observed in a see-through
fashion via the bonding surfaces of the transparent base members 22
and 23 and an optical devices that directs an enlarged virtual
image of the image displayed on the image display element 11 to the
observer's eye. The eyepiece optical system 21 has a
non-axisymmetric positive optical power so as to satisfactorily
correct the aberrations of the image light that has entered it.
[0047] The transparent base members 22 and 23 are formed of, for
example, acrylic resin, and are joined together with adhesive 25
(see FIG. 5F). Here, the transparent base member 22 is a
plane-parallel plate of which a bottom-end part is made
increasingly thin toward the bottom end thereof so as to be shaped
like a wedge and of which a top-end part is made increasingly thick
toward the top end thereof. The transparent base member 23 is a
plane-parallel plate of which a top-end part is so shaped as to fit
the bottom-end portion of the transparent base member 22 so that
the transparent base members 22 and 23 together form substantially
a plane-parallel plate.
[0048] If the transparent base members 22 and 23 are not joined
together, the light of the outside-world image is refracted when it
passes the wedge-shaped bottom-end portion of the transparent base
member 22. This produces distortion in the outside-world image
observed via the transparent base member 22. By contrast, when the
transparent base members 22 and 23 are joined together so as to
together form substantially a plane-parallel plate, the refraction
that the light of the outside-world image undergoes when it passes
through the wedge-shaped bottom-end part of the transparent base
member 22 is cancelled with the transparent base member 23. This
helps prevent distortion from being produced in the outside-world
image observed in a see-through fashion.
[0049] The optical element 24 is built with a volume-phase-type
reflective hologram that diffracts light in three wavelength bands
of, for example, 465.+-.10 nm, 520.+-.10 nm, and 635.+-.10 nm that
is incident thereon at a prescribed angle of incidence. The optical
element 24 is bonded to the slanted surface of the bottom-end
portion of the transparent base member 22, and thus the optical
element 24 is held between the transparent base members 22 and 23.
The transmittance of the optical element 24 is set to be 10% or
more.
[0050] The optical element 24 is formed of a hologram
photosensitive material 24a (see FIG. 1) such as a photopolymer, a
silver-halide-based material, or gelatin bichromate. Among the just
mentioned materials, a photopolymer is particularly preferable
because it can be produced by a dry process.
[0051] In the image display apparatus 1 structured as described
above, the light emitted from the light source 12 of the image
display element 11 is diffused by the one-directional diffuser
plate 13, is then condensed by the condenser lens 14, and is then
incident on the LCD 15. The light incident on the LCD 15 is
modulated according to an image signal, and then exits, as an image
light, from the LCD 15. Here, the LCD 15 displays an image
itself.
[0052] The image light from the LCD 15 enters the transparent base
member 22 of the eyepiece optical system 21 via the top-end surface
thereof, and is then totally reflected a plurality of times on the
mutually opposite surfaces thereof so as to be incident on the
optical element 24. The light incident on the optical element 24 is
reflected thereon so as to reach the optical pupil E. At the
position of the optical pupil E, the observer observes an enlarged
virtual image of the image displayed on the LCD 15. The distance
from the optical pupil E to the virtual image is about several
meters, and the size of the virtual image is ten or more times as
large as the image displayed on the LCD 15.
[0053] On the other hand, the transparent base members 22 and 23
and the optical element 24 transmit most of the light from the
outside world, and thus permit the observer to observe the
outside-world image. Thus, the virtual image of the image displayed
on the LCD 15 is observed overlaid on part of the outside-world
image. As will be understood from the foregoing, the optical
element 24 can be said to function as a combiner that
simultaneously directs the image formed by the image display
element 11 and the outside-world image to the observer's eye.
[0054] As described above, the image display apparatus 1 is so
structured that the image light exiting from the LCD 15 is directed
to the optical element 24 by being totally reflected within the
transparent base member 22. This makes it possible to arrange the
image display element 11 far away from immediately before the
observer's eye, and thereby permits the observer to observe the
outside world via a wide field of view. Moreover, it is possible to
make the transparent base members 22 and 23 as thin as about 3 mm,
like common eyeglass lenses, and thereby to make the image display
apparatus 1 compact and lightweight.
[0055] Moreover, since the optical element 24 diffracts only light
of prescribed wavelengths that is incident thereon at a prescribed
angle of incidence, it does not affect the light of the
outside-world image that is transmitted through the transparent
base members 22 and 23 and the optical element 24. Thus, the
observer can as usual observe the outside-world image via the
transparent base members 22 and 23 and the optical element 24.
Moreover, since the transmittance of the optical element 24 is set
to be 10% or more, the observer can observe the outside-world image
sufficiently clearly via the transparent base members 22 and 23 and
the optical element 24.
3. Transparent Base Members
[0056] Next, the transparent base members 22 and 23 will be
described in detail. FIG. 5A is a plan view of the transparent base
member 22 (the first transparent base member), and FIG. 5B is a
front view of the transparent base member 22. FIG. 5C is a plan
view of the transparent base member 23 (the second transparent base
member), and FIG. 5D is a front view of the transparent base member
23. FIG. 5E is a plan view of the eyepiece optical system 21 having
the transparent base members 22 and 23 joined together. FIG. 5 is a
sectional view of the transparent base members 22 and 23, taken
around the joint surfaces thereof.
[0057] The transparent base member 22 as a whole has the shape of a
truncated rectangular pyramid, with the top and bottom surfaces
thereof joined by four side surfaces. These four side surfaces are
surfaces 22a, 22b, 22c, and 22d located counter-clockwise around
the top surface. These surfaces 22a, 22b, 22c, and 22d are so
oriented that the lines normal thereto point in mutually different
directions. One of these surfaces (for example, the surface 22d)
has part thereof formed into a protruding portion 22e that
protrudes upward from the top surface. The optical element 24 is
bonded to, for example, the surface 22b of the transparent base
member 22.
[0058] On the other hand, the transparent base member 23 is so
shaped that, when the transparent base member 22 is joined thereto,
they together form a plane-parallel plate. That is, the transparent
base member 23 has the shape of a plane-parallel plate from which
the shape of the transparent base member 22 has been removed. Here,
the surfaces of the transparent base member 23 that face the
surfaces 22a, 22b, and 22c of the transparent base member 22 when
the transparent base members 22 and 23 are joined together are
called the surfaces 23a, 23b, and 23c, respectively. These surfaces
23a, 23b, and 23c are so oriented that the lines normal thereto
point in mutually different directions.
[0059] In this way, to one transparent base member 22 having the
optical element 24 bonded thereto, the other transparent base
member 23 is joined with the adhesive 25 so that the optical
element 24 is held in between, and thereby the eyepiece optical
system 21 shown in FIG. 5E is formed. Seen in a plan view, the
eyepiece optical system 21 is shaped like an eyeglass lens. With
this eyepiece optical system 21, the outside-world image can be
observed in a see-through fashion via the joint surfaces (the
surfaces 22a, 22b, 22c, 23a, 23b, and 23c) of the transparent base
members 22 and 23.
4. Production Procedure of the Eyepiece Optical System
[0060] Next, the production procedure of the eyepiece optical
system 21 as an optical device will be described. The production
procedure of the eyepiece optical system 21 involves the following
five processes: a bonding process, an exposure process, a fixing
process, a baking (heat treatment) process, and a joining process.
If the production of the eyepiece optical system 21 through these
processes is called "fabrication" of the optical element 24, the
use of the thus fabricated optical element 24 in various devices
can be called "reproduction" thereof. Now, the above-mentioned
production procedure will be described in detail with reference to
FIG. 1.
[0061] First, on one transparent base member 22 to be used during
reproduction, a hologram photosensitive material 24a, for example a
photopolymer, is bonded (the bonding process). Then, by two-beam
interference of laser light, the hologram photosensitive material
24a on the transparent base member 22 is exposed (the exposure
process). Subsequently, the hologram photosensitive material 24a is
irradiated with ultraviolet rays so as to be fixed (the fixing
process).
[0062] Then, the hologram photosensitive material 24a bonded on the
transparent base member 22 is baked to form a hologram (the optical
element 24) with high diffraction efficiency. Then, lastly, on the
surface of the transparent base member 22 at which it will be
joined to the other transparent base member 23, ultraviolet-curing
adhesive, which is a kind of light-curing adhesive, is applied as
the adhesive 25 (see FIG. 5F), and is then irradiated with
ultraviolet rays so as to be cured. Thus, the transparent base
members 22 and 23 are joined together with the hologram
photosensitive material 24a (optical element 24) held between them
(the joining process). In this way, the eyepiece optical system 21
is formed.
[0063] Incidentally, the reason that the diffraction efficiency of
the hologram increases in the baking process is as follows.
Exposing the hologram photosensitive material 24a produces
interference fringes, forming high- and low-refractive-index
portions in the hologram. However, since the photopolymer used as
the hologram photosensitive material 24a is a polymer material,
simply exposing it does not provide a sufficiently large difference
in refractive index between the high- and low-refractive-index
portions. Here, conveniently, when heat is applied to the hologram
photosensitive material 24a in the baking process, unreacted
monomers and the like in the hologram photosensitive material 24a
are diffused by the heat, producing a large difference in density.
This increases the difference in refractive index within the
hologram, and thus increases the diffraction efficiency
thereof.
[0064] The baking process may be performed after the joining
process. In that case, part of the adhesive 25 that remains uncured
after joining may adversely affect the hologram layer. For this
reason, it is preferable that, as in the embodiment under
discussion, baking be completed before joining.
5. Details of the Eyepiece Optical System
[0065] In the embodiment under discussion, when the eyepiece
optical system 21 is produced, the hologram photosensitive material
24a and the transparent base members 22 and 23 are formed of an
acrylic material. Specifically, the hologram photosensitive
material 24a is formed of a material containing many acrylate
derivatives such as polymethyl methacrylate (PMMA), methacrylate,
phenoxyethyl acrylate, chlorophenyl acrylate triethyleneglycol
diacrylate, and trimethylolpropane trimethacrylate (an example of
such a material includes "OmniDex" manufactured by DuPont). On the
other hand, the transparent base members 22 and 23 are formed of
metyacrylic resin such as polymethyl methacrylate (PMMA) (examples
of such materials include "ACRYPET" manufactured by Mitsubishi
Rayon Co., Ltd, "Zeonex" manufactured by Nippon Zeon Co., Ltd., and
"Delpet" manufactured by Asahi Kasei Corporation.
[0066] When acrylic materials are used as the hologram
photosensitive material 24a, that is, the material of the optical
element 24, and as the material of the transparent base member 22
to which the hologram photosensitive material 24a is bonded, higher
adhesion is obtained between the hologram photosensitive material
24a and the transparent base member 22 than when those are formed
of different kinds of material. Thus, without performing special
processing as by adding another substance to the hologram
photosensitive material 24a or treating the surface of the
transparent base member 22 as conventionally practiced, it is
possible to obtain increased adhesion between them.
[0067] An acrylic material used as the hologram photosensitive
material 24a provides excellent properties as a hologram material;
specifically, it exhibits high polymerization reactivity (in terms
of reaction rate and exposure sensitivity) during laser exposure in
the exposure process, and exhibits quick refractive index change
after laser exposure. This makes it possible to record interference
fringes in a short time, and thus helps avoid the influence of
ambient factors such as vibration. In addition, the thus recorded
hologram provides a large difference in refractive index, making it
possible to fabricate a hologram with excellent properties such as
high diffraction efficiency.
[0068] On the other hand, the transparent base members 22 and 23
have properties such as being highly transparent and being easily
moldable by injection molding or the like, and thus offers
excellent properties as an optical base material. In addition, the
transparent base members 22 and 23 are inexpensive and lightweight,
and more securely absorbs shock and external pressure than glass.
Thus, even when the eyepiece optical system 21 is built with the
transparent base members 22 and 23 and is used as a combiner in the
HMD as in this embodiment, it is possible to achieve high safety to
the observer's eye.
[0069] As discussed above, in the production of the eyepiece
optical system 21, using acrylic materials as both the hologram
photosensitive material 24a and the transparent base members 22 and
23 is extremely effective, because doing so offers not only
increased adhesion but may other advantages.
[0070] Moreover, in this embodiment, since the eyepiece optical
system 21 is formed by joining the transparent base member 22 to
the other transparent base member 23 formed of an acrylic material
with the optical element 24 held between them, the eyepiece optical
system 21 can easily be used as a combiner in the HMD.
[0071] Here, it is preferable that the adhesive 25 used to join the
transparent base members 22 and 23 together be formed of an acrylic
material. Examples of such adhesives include those containing an
acrylate derivative such as an acrylic denatured oligomer,
tetrahydrofurfuryl methacrylate, substituted ethyl acrylate,
substituted urethane acrylate (for example, "LCR0628A" manufactured
by Toagosei Co., Ltd. and "NOA 76" manufactured by Norland Products
Inc.).
[0072] In general, the adhesion (bonding strength) of a material is
higher against a material of a similar kind than against one of a
different kind. Thus, when the hologram photosensitive material 24a
and the transparent base members 22 and 23 are both formed of an
acrylic material, if an acrylic material is used also as the
adhesive 25, it is naturally possible to obtain higher
adhesion.
[0073] It is preferable that the adhesive 25 used to join the
transparent base members 22 and 23 together be, among different
kinds of acrylic adhesive, of an ultraviolet-curing type. An
ultraviolet-curing adhesive has a low contraction coefficient, and
thus its use helps minimize deformation of and damage to the
optical element 24 and the transparent base members 22 and 23.
Moreover, unlike a thermosetting adhesive, an ultraviolet-curing
adhesive does not require heat to cure, and thus its use helps
prevent deformation of the transparent base members 22 and 23 under
heat. Furthermore, an ultraviolet-curing adhesive of a type that
contains no solvent is used to prevent the optical element 24 from
being adversely affected by a solvent.
[0074] In this embodiment, the optical element 24 is formed by
bonding the hologram photosensitive material 24a in an unexposed
state to the transparent base member 22 to be used during
reproduction and then exposing it to laser light. This, as compared
with a procedure involving re-bonding of the optical element 24
after laser exposure to the transparent base member to be used
during reproduction, not only requires less fabrication processes
and thus contributes to higher productivity, but also offers the
following various advantages. There is no need to use adhesive for
re-bonding, and therefore the optical element 24 is prevented from
being adversely affected by adhesive. There is no likeliness of the
optical element 24 being re-bonded into a deviated position. Even
if the surface accuracy of the transparent base member 22 is
slightly deviated from what it should ideally be, the optical
element 24 can be fabricated by performing exposure with the
deviation taken into consideration. Thus, it is possible to
eliminate or minimize the influence of the deviation of the surface
accuracy of the transparent base member 22.
[0075] Moreover, since the hologram photosensitive material 24a in
an unexposed state is bonded to the transparent base member 22 to
be used during reproduction and is then exposed to laser light to
fabricate an optical device having a hologram recorded thereon, the
polymerization reaction of the hologram photosensitive material 24a
during exposure and the subsequent polymerization reaction during
fixing permit the surface 22a to adhere firmly on the transparent
base member 22. This effect is augmented by the fact that the
hologram photosensitive material 24a and the transparent base
member 22 are formed of similar kinds of material (acrylic
materials). The hologram photosensitive material 24a in an
unexposed state is bonded, by its own adhesion, to the transparent
base member 22, but it loses its adhesion after exposure. Thus, the
optical element 24 can be said to be bonded to the transparent base
member 22 through the polymerization reaction that takes place in
the exposure process in which the hologram photosensitive material
24a is exposed to laser light and in the fixing process in which
the hologram photosensitive material 24a is fixed by being
irradiated with light.
[0076] In a long run, during reproduction, the optical element 24
formed as a hologram slightly deteriorates and becomes yellowish
under ultraviolet rays. To avoid this, for example, an ultraviolet
absorber is added to the transparent base members 22 and 23 formed
of an acrylic material so that they reduce the ultraviolet rays
that reach the optical element 24. To alleviate the deterioration
and yellowing of the optical element 24 under ultraviolet rays as
just described, it is preferable that the spectral transmittance of
the transparent base members 22 and 23 at a wavelength of 360 nm be
10% or less. This applies also when the transparent base members 22
and 23 contain no ultraviolet absorber.
[0077] Adding too much ultraviolet absorber to the transparent base
members 22 and 23 makes them appear yellowish by themselves. This
is undesirable not only because of the resulting poor appearance,
considering that this embodiment is actually used with the eyepiece
optical system 21 as a combiner located before the eye, but also
because the observer then cannot properly recognize the colors of
the outside-world image observed in a see-through fashion via the
eyepiece optical system 21 (that is, the see-through property
degrades). To avoid this by keeping the transparent base members 22
and 23 transparent and obtaining a satisfactory see-through
property, it is preferable that the spectral transmittance of the
transparent base members 22 and 23 at a wavelength of 400 nm be set
to be 80% or more, and that the content of the ultraviolet
absorbent be so reduced as to achieve such a spectral
transmittance.
[0078] It is alternatively possible to reduce the deterioration and
yellowing of the transparent base members 22 and 23 by eliminating
light in the ultraviolet region from the light emitted from the
light source used during reproduction.
[0079] Baking the hologram photosensitive material 24a after it has
been exposed to laser light helps increase the diffraction
efficiency of the optical element 24. This is because, as described
previously, heating permits unreactd monomers contained in the
hologram photosensitive material 24a to diffuse and move.
Accordingly, it is preferable that the refractoriness-under-load
temperature (deflection temperature under load) of the transparent
base member 22 be as high as possible, and at least higher than or
equal to the temperature that permits unreacted monomers contained
in the hologram photosensitive material 24a to diffuse and move
within the hologram photosensitive material 24a. Here, the
refractoriness-under-load temperature denotes the temperature at
which the transparent base member 22 softens (deforms) under
load.
[0080] In this embodiment, the refractoriness-under-load
temperature of the transparent base member 22 is set to be about
20.degree. C. or more higher than the temperature that permits
unreacted monomers to diffuse and move. The temperature that
permits unreacted monomers to diffuse and move varies from one type
of hologram photosensitive material 24a to another. In a case
where, for example, "OmniDex" manufactured by DuPont is used, it is
advisable to use, as the transparent base member 22, one having a
refractoriness-under-load temperature of 100.degree. C. or higher.
Using such a transparent base member 22 makes it possible to avoid
deformation of the transparent base member 22 in the baking process
while simultaneously permitting unreacted monomers to diffuse so as
to increase the local difference in refractive index and thereby
increase diffraction efficiency. Moreover, since deformation of the
transparent base member 22 can be prevented, it is possible to
obtain satisfactory surface accuracy.
[0081] The wavelength (reproduction wavelength) of the light
(reproduction light) that exits from the optical element 24 during
reproduction is determined by the wavelength of the laser light to
which the hologram photosensitive material 24a is exposed during
fabrication. Accordingly, to obtain single-color reproduction light
during reproduction, it is necessary to expose the hologram
photosensitive material 24a to laser light of at least one color.
To obtain colored reproduction light during reproduction, it is
necessary to expose the hologram photosensitive material 24a to
laser light of a plurality of wavelengths corresponding to
necessary colors. From the perspective of enjoying images, colored
reproduction is preferable, and accordingly, in this embodiment,
the hologram photosensitive material 24a is exposed to laser light
of three wavelengths corresponding to red (R), green (G), and blue
(B) during fabrication so that color images (reproduced images) are
obtained during reproduction.
[0082] Here, to obtain bright colored reproduced images, the
diffraction efficiency of the optical element 24 needs to be
increased at each of the three, namely R, G, and B, wavelengths.
The diffraction efficiency indicates what portion of the energy of
incident light can be extracted as the energy of diffracted light,
and is generally expressed by the ratio, as given in percentage, of
the intensity of diffracted light of a particular order to the
intensity of incident light.
[0083] With an optical element 24 that has a diffraction peak for
only one color (a peak in diffraction efficiency), that is, only
one diffraction peak, it is in principle impossible to obtain
diffraction efficiency of 100% or higher. By contrast, with a color
hologram that diffracts light at a plurality of wavelengths
(diffraction wavelengths), diffraction peaks exist one for each of
the different wavelengths, and thus it is possible, for example, to
make the sum of the diffraction efficiency at those different
wavelengths equal to or higher than 100%.
[0084] Specifically, in this embodiment, the optical element 24 is
so fabricated that it has a plurality of diffraction peaks
corresponding to a plurality of wavelengths (RGB) and that the sum
of the diffraction efficiency at those peaks is equal to or higher
than 100%. Such a optical element 24 can be realized by, during its
fabrication, exposing the hologram photosensitive material 24a to
RGB laser light and then increasing the diffraction efficiency for
each color in the baking process.
[0085] When the eyepiece optical system 21 incorporating the above
optical element 24 is used as a combiner in the HMD, and a color
image is displayed on the image display apparatus 1 of the HMD, the
observer can observe, as a virtual image, a bright color image via
the optical element 24. Moreover, the light from the light source
12 (reproduction light source) used during reproduction can be
effectively used. Moreover, since the wavelengths at which the
individual diffraction peaks are located are the wavelengths
corresponding to the individual R, G, and B colors, it is possible
to present the observer with, as a virtual image, a color image
with high color purity and a wide color reproduction range.
[0086] Ideally, the diffraction efficiency at the diffraction peak
of each of the R, G, and B wavelengths is 100% at the maximum, and
therefore, ideally, its sum equals 100% multiplied by the number of
diffraction peaks. In reality, however, the hologram photosensitive
material 24a is sensitive to laser light at a plurality of
wavelengths, and the sensitivity at those different wavelengths
affect one another, making it difficult to obtain the maximum
diffraction efficiency of 100% at all the diffraction peaks. In
fact, the diffraction efficiency at the diffraction peak of each of
the wavelengths is, for example, about several tens percent (for
example, about 50% at each of the R, G, and B wavelengths as shown
in FIG. 6A). Even then, it is still possible to make the sum of the
diffraction efficiency at the diffraction peaks of the different
wavelength equal to or higher than 100% (in the example shown in
FIG. 6A, 150% or higher).
[0087] To obtain satisfactory color display, it is necessary to
strike a proper brightness balance (color balance) among the
different colors (RGB). In a simplified form, the brightness of an
image is calculated as the sum of the "diffraction efficiency
multiplied by the intensity of illumination light (reproduction
light) at the same wavelength as that of diffracted light
(diffraction wavelength)" for the different colors. A proper color
balance in the image is achieved by adjusting the values of the
just mentioned "diffraction efficiency multiplied by the intensity
of illumination light at the same wavelength as that of diffracted
light" for the different colors so that those values are in a
prescribed ratio that produces satisfactory white display.
Accordingly, in reality, the just mentioned ratio takes a fixed
value that varies with the diffraction wavelength. That is, to
obtain satisfactory color display, the diffraction efficiency at
the different diffraction wavelengths needs to be set in
consideration of the intensity of illumination light at the same
wavelengths as those diffraction wavelengths, respectively.
[0088] For example, suppose that, in the optical element 24 that
produces diffracted light of three, namely R, G, and B, colors, the
diffraction efficiency for those three colors is approximately
equal as shown in FIG. 6A. Suppose also that, when this optical
element 24 is illuminated with an illumination light source (the
light source 12 as a reproduction light source), the intensity of R
light is insufficient to obtain white display. In this case, it is
advisable to make the diffraction efficiency for R light higher
than that for other light as shown in FIG. 6C.
[0089] In FIG. 6B, the curves "r", "g", and "b" represent the
intensity of R, G, and B light, respectively, and the curve "L"
represents the overall intensity of R, G, and B light. Here, the
light intensity is plotted, for example, in terms of intensity
relative to that of B light.
[0090] As discussed above, by adjusting the balance of the amounts
of R, G, and B diffracted light so as to obtain white display, it
is possible to obtain satisfactory color display. From the
perspectives of the brightness of the image and efficient use of
the light of the reproduction light source, it is advisable to
fabricate (in particular, bake) the optical element 24 so that, as
shown in FIG. 6C, the maximum value of the diffraction efficiency
among those at the different wavelengths (R, G, and B) at which the
diffraction efficiency has a peak, is 70% or higher.
[0091] As described above, the diffraction efficiency of the
hologram photosensitive material 24a increases in the baking
process (its sensitivity is increased). On the other hand, in this
embodiment, the transparent base member 22 is formed of an acrylic
material, which is not highly resistant to heat. For this reason,
if the hologram photosensitive material 24a is baked at a baking
temperature of 100.degree. or higher as commonly practiced, the
transparent base member 22 deforms. Thus, the hologram
photosensitive material 24a cannot be baked at the just mentioned
baking temperature. However, if the hologram photosensitive
material 24a is not baked at all, it is not possible to "make the
sum of the diffraction efficiency for R, G, and B light equal to or
higher than 100%", nor is it possible to "make the maximum value of
the diffraction efficiency among those for R, G, and B color equal
to or higher than 70%".
[0092] Thus, in the structure of this embodiment where the
transparent base member 22 is formed of an acrylic material, the
basing process is performed under milder conditions. That is, it is
advisable to bake the hologram photosensitive material 24a at a
lower temperature but for a longer period.
[0093] Specifically, it is advisable to perform baking at a
temperature equal to or lower than the refractoriness-under-load
temperature of the transparent base member 22. To obtain the effect
of baking efficiently in a short period, the higher the baking
temperature, the better. For example, let the
refractoriness-under-load temperature be T .degree. C., then it is
advisable to perform baking at a baking temperature (.degree. C.)
of T-.DELTA.
[0094] (where .DELTA. is one of the values 5, 10, 15, 20, 25, and
30).
[0095] Thus, when the transparent base member 22 is formed of an
acrylic material of a common grade, though depending on the heat
resistance of the acrylic material, the typical baking temperature
is 100.degree. or lower, fulfilling the above formula. Needless to
say, when the transparent base member 22 is formed of an acrylic
material with higher heat resistance, the baking temperature as
calculated based on the above formula is 100.degree. C. or higher,
and thus the baking process can be performed at a temperature of
100.degree. C. or higher.
[0096] In this embodiment, the eyepiece optical system 21 has been
described as having the optical element 24 held between the
transparent base members 22 and 23. Needless to say, the structure
of this embodiment may be applied also in a case where the optical
device is so structured as to have the optical element 24 simply
bonded on the transparent base member 22. In this case, there is no
need to use adhesive 25 as used in this embodiment to join the
transparent base members 22 and 23 together, and thus it is
possible to prevent the optical element 24 from being adversely
affected by adhesive 25.
[0097] In this embodiment, the transparent base members 22 and 23
have been described as having flat joint surfaces. Instead, the
joint surfaces may be, for example, curved.
[0098] In this embodiment, the image display apparatus 1 has been
described as being applied to an HMD. Instead, it may be applied
to, for example, a head-up display.
[0099] In this embodiment, the transparent base members 22 and 23
have been described as being flat-plate-shaped. Instead, they may
have curvatures. In that case, the eyepiece optical system 21 can
function as an eyeglass lens for correcting the dioptric power of
the eye.
[0100] As described above, according to the present invention, an
optical device is produced by bonding an optical element formed as
a hologram on a transparent base member, and the optical element
and the transparent base member are both formed of an acrylic
material.
[0101] Thus, without performing special processing on either of the
optical element and the transparent base member, it is possible to
obtain increased adhesion between them. Moroever, since the
transparent base member is formed of an acrylic material, even when
the optical device of the present invention is used as a combiner
in a head-mounted display, it is possible to achieve higher safety
to the eye of the observer wearing the head-mounted display.
Furthermore, advantageously, the optical element formed of an
acrylic material offers excellent properties (in terms of
sensitivity, refractive index change, etc.) as a hologram, and the
transparent base member formed of an acrylic material offers high
transmittance, is easy to old, and offers excellent properties as
an optical base member.
[0102] The above transparent base member may be joined to another
transparent base member so that the above optical element is held
in between. In that case, the optical device according to the
present invention can easily be applied as a combiner in a
head-mounted display. Moreover, in such a head-mounted display, the
distortion produced in the light of the outside-world image when it
passes through one transparent base member can be cancelled with
the other transparent base member, and thus it is possible to
prevent the outside-world image from being distorted.
[0103] It is preferable that the transparent base members be joined
together with adhesive formed of an acrylic material. In that case,
the optical element, the transparent base members, and the adhesive
are all formed of similar kinds of material, namely acrylic
materials. Thus, even with a structure where the transparent base
members are joined together with adhesive, higher adhesion is
obtained between them.
[0104] Here, it is preferable that the adhesive is of a
ultraviolet-curing type. An ultraviolet-curing adhesive has a low
contraction coefficient, and thus its use helps minimize
deformation of and damage to the optical element and the
transparent base members. Moreover, unlike a thermosetting
adhesive, an ultraviolet-curing adhesive does not require heat to
cure, and thus its use helps prevent deformation of the transparent
base members under heat. Furthermore, an ultraviolet-curing
adhesive of a type that contains no solvent is used to prevent the
optical element 24 from being adversely affected by a solvent.
[0105] It is preferable that the optical element be formed by
bonding a hologram photosensitive material (formed of an acrylic
material) in an unexposed state to the transparent base member to
be used during reproduction and then exposing it to laser light.
Here, "reproduction" denotes occasions on which the fabricated
optical element is used in various devices. That is, using the
optical element "during reproduction" is a different concept from
using it "during fabrication".
[0106] This structure, where the hologram photosensitive material
in an unexposed state is bonded to the transparent base member to
be used during reproduction and is then exposed to laser light, as
compared with a structure where the optical element after laser
exposure is re-bonded to the transparent base member to be used
during reproduction, requires less fabrication processes and thus
contributes to higher productivity. Moreover, There is no need to
use adhesive for re-bonding, and therefore the optical element is
prevented from being adversely affected by adhesive. Furthermore,
there is no likeliness of the optical element being re-bonded into
a deviated position. In this way, many advantages are obtained.
[0107] It is preferable that the optical element be bonded to the
transparent base member through the polymerization reaction that
takes place in the exposure process in which the hologram
photosensitive material (formed of an acrylic material) is exposed
to laser light and in the fixing process in which the hologram
photosensitive material is fixed by being irradiated with light.
(for example, ultraviolet rays). The hologram photosensitive
material in an unexposed state is bonded by its own adhesion.
However, by exploiting the polymerization reaction that takes place
in the exposure and fixing processes in this way, it is possible to
more firmly join the hologram photosensitive material and the
transparent base member together.
[0108] It is preferable that the spectral transmittance of the
transparent base member at a wavelength of 360 nm be 10% or less.
In that case, ultraviolet rays in a short-wavelength region are
mostly absorbed by the transparent base member. In the long run,
the optical element slightly deteriorates and become yellowish
under ultraviolet rays. With the above structure, however, it is
possible to prevent deterioration of the optical element.
[0109] It is preferable that the spectral transmittance of the
transparent base member at a wavelength of 400 nm be 80% or more.
In that case, it is possible to make the transparent base member
satisfactorily transparent and thereby obtain a satisfactory
see-through property.
[0110] It is preferable that the refractoriness-under-load
temperature of the transparent base member be set to be equal to or
higher than the temperature that permits unreacted monomers in the
hologram photosensitive material to diffuse and move within the
hologram photosensitive material. Here, the
refractoriness-under-load temperature denotes the temperature at
which the transparent base member softens (deforms) under load. In
that case, it possible to avoid deformation of the transparent base
member in the baking process while simultaneously permitting
unreacted monomers to diffuse so as to increase the local
difference in refractive index and thereby increase diffraction
efficiency. Moreover, since deformation of the transparent base
member can be prevented, it is possible to obtain satisfactory
surface accuracy.
[0111] It is preferable that the optical element have a plurality
of diffraction efficiency peaks corresponding to a plurality of
wavelengths, and that the sum of the diffraction efficiency at the
wavelengths at which the diffraction efficiency has peaks be 100%
or more. In that case, for example, when an image from an image
display element is presented as a virtual image to an observer via
the optical device according to the present invention, it is
possible to present, as the virtual image, a bright color image.
Moreover, it is also possible to more efficiently use the light of
the light source of the image display element.
[0112] Here, it is preferable that the plurality of wavelengths be
those corresponding to red (R), green (G), and blue (B) colors,
respectively. In that case, with the R, G, and B colors, it is
possible to obtain a color image with high color purity and a wide
color reproduction rage.
[0113] It is preferable that the maximum value of the diffraction
efficiency among those at the wavelengths at which the diffraction
efficiency has peaks be 70% or more. In that case, it is possible
to obtain a bright image, and to use the light of the light source
highly efficiently.
[0114] According to the present invention, an image display
apparatus is provided with the above-described optical device
according to the present invention and an image display element
that displays an image to feed it to the optical device. With this
structure, the observer can observe, via the optical device, the
image fed from the image display element and simultaneously
observe, also via the optical device but here in a see-through
fashion, the outside-world image.
[0115] Here, it is preferable that the optical element included in
the optical device be a volume-phase-type reflective hologram In
that case, by making the hologram reflect the image light fed from
the image display element toward the observer, it is possible to
permit the observer to observe a virtual image. In addition, since
a volume-phase-type reflective hologram exhibits high transmittance
to the light of the outside-world image, the observer can observe
the outside-world image clearly.
[0116] The optical element included in the optical device may be a
combiner that directs the image fed from the image display element
and the outside-world image simultaneously to the observer's eye.
In that case, via the optical element, the observer can observe the
image fed from the image display element and the outside-world
image.
[0117] The optical device may form an eyepiece optical system that
directs an enlarged virtual image of the image displayed on the
image display element to the observer's eye. In that case, the
observer can satisfactorily clearly observe, as a virtual image,
the image displayed on the image display element. Moreover, since
the eyepiece optical system presents the observer with, as a
virtual image, the image displayed on the image display element, it
is possible to make the optical device forming the eyepiece optical
system compact and lightweight, and thus to make the image display
apparatus compact and lightweight.
[0118] It is preferable that the eyepiece optical system have a
non-axisymmetric (positive) optical power. In that case, even when
the eyepiece optical system is made compact, it is possible to
present the observer with an image with satisfactorily corrected
aberrations.
[0119] It is preferable that the transparent base member of the
optical device be so structured as to totally reflect within itself
the light of the image fed from the image display element to direct
it to the optical element. With this structure, it is possible to
efficiently use the image light fed from the image display element
and thereby present the observer with a bright image. Moreover, it
is possible to arrange the image display element away from the
optical device, and thus to permit the observer to observe the
outside world via a wide field of view.
[0120] It is preferable that the transmittance of the optical
element of the optical device be 10% or more. In that case, even
via the optical element, the observer can observe the outside-world
image satisfactorily clearly in a see-through fashion.
[0121] According to the present invention, a head-mounted display
is provided with the above-described image display apparatus and a
supporter that supports the image display apparatus before an
observer's eye. With this structure, since the image display
apparatus is supported before the observer's eye by the supporter,
the observer has his or her hands free, and can thus observe the
outside-world image and, as a virtual image, the image displayed on
the image display element while doing handwork with his or her free
hands. Moreover, the observation direction of the observer is fixed
in one direction, and therefore, advantageously, the observer can
easily find the displayed ed image even in a dark environment.
[0122] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims, the invention may be practiced other than as specifically
described.
* * * * *